Leo was crowned co-emperor by his father in 751, and was married to the Athenian Irene in 769. In 780 he succeeded his father as sole emperor.
In 776 he associated his young son, Constantine, with himself in the empire, and suppressed the first in a series of uprisings led by his half-brothers Christopher and Nikephoros which broke out as a result. The failed claimants to the throne were blinded, tonsured , and exiled.
During his short reign, Leo fought against the Abbasid Caliphate under Al-Mahdi . He dispatched forces into Syria under Michael Lachanodrakon in 776 and 778. Nevertheless, the Abbasid forces succeeded in raiding into Asia Minor in 776, 779, and 780.
Unlike his father and grandfather, Leo showed himself relatively tolerant towards iconodules , and restored an iconophile patriarch . Only at the very end of his reign, in 780, did he have a number of iconodule officials tortured and imprisoned. Following the precedent set by his father, he prepared an expedition against Kardam of Bulgaria , but died before achieving anything of significance.
During his reign Leo was largely under the influence of his wife Irene , and when he died suddenly in 780 she was left as the guardian of his son and successor, Constantine VI .